fix(3p/nix): Link to Abseil built by Nix

This didn't work previously ... but now it does. I think setting the
standard explicitly is what did the trick, but it's slightly unclear
to me why.

Either way this means that Abseil is no longer constantly getting
recompiled when building Nix, which is nice.

Change-Id: I377f7b68bf1ef9045df6a2eee8fdd0c92f243547
Reviewed-on: https://cl.tvl.fyi/c/depot/+/921
Tested-by: BuildkiteCI
Reviewed-by: lukegb <lukegb@tvl.fyi>

third_party/nix/CMakeLists.txt

@@ -20,24 +20,7 @@ find_package(CURL)
 find_package(SQLite3)
 find_package(Threads)
 find_package(LibLZMA)
-
-# Abseil really doesn't like being precompiled. It is included here as
-# an imported CMake project (i.e. it will be built /with/ this
-# project).
-#
-# In development mode, the 'abseil_cpp' folder is symlinked to
-# ../abseil_cpp (the relative location in the depot). In derivation
-# builds this symlink is automatically replaced with an appropriate
-# link to the Abseil sources in the Nix store.
-add_subdirectory(abseil_cpp)
-install(DIRECTORY abseil_cpp/absl
-  DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
-  FILES_MATCHING
-    PATTERN "*.inc"
-    PATTERN "*.h"
-    PATTERN "copts" EXCLUDE
-    PATTERN "testdata" EXCLUDE
-  )
+find_package(absl REQUIRED)
 
 # Linking precompiled glog binaries (linked against libstdc++) leads
 # to working binaries that do not output log messages. It seems that